The preliminary U.S. proposal came as a confidential U.N. report described an Iranian initiative that had multiplied Tehran’s stockpile of near-bomb-grade uranium.

In a confidential report seen by the BBC, the International Atomic Energy Agency (IAEA) said Iran now possesses over 400kg of uranium enriched to 60% purity - well above the level used for civilian purposes and close to weapons grade, and a near 50% increase in three months.